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Abstract

We present an improved theoretical model to estimate the minimum fiber length required for achieving a desired degree of wavefront filtering in stellar interferometry. The proposed model is based on modal analysis of the fiber and is compared with numerical results obtained through the beam propagation method as well as with reported experimental observations. We also study the effect of introducing a spatial filter at the output end of the fiber and show that the required fiber length can be reduced significantly by introducing a circular aperture of optimum radius after the fiber.

Figures (6)

Schematic representation of the modal power along the fiber length (a) FM (b-d) Modes whose overlap with the input field is > 1% (e,f) HOMs exhibiting very low confinement losses. The y-axis shows the power of the various modes such that the power is proportional to the coupling efficiency of the respective modes. The total power is assumed to be 1, which is the sum of the coupling efficiencies of all the modes. Inset: Cross-section of the fibre structure

Loss and coupling efficiency of the 12 cladding modes with coupling efficiency > 0.1%. The circles and crosses represent the coupling efficiency and confinement loss, respectively. Note that the number of dots and crosses seem less than 12 in the figure because some of the modes are degenerate with similar values of coupling efficiency and confinement loss, and thus the dots and crosses overlap for some of the modes. Modal effective index = propagation constant of the mode / free space wave vector of light

Evolution of the total power (normalized to the input power) and power in the FM (normalized to the local power) along the fiber length. The right-hand side figure shows the evolution of the modal profile along the fiber length. X and Z denote the radial distance and direction of propagation, respectively. The region shaded in light green is the core of the fiber. The regions colored in yellow and dark green are the cladding, and the outermost region shaded in red is the absorption coating.

The effect of varying the radius of a circular aperture placed at the output end of the single-mode fiber. There exits a trade-off between improving the filtering capability and obtaining maximal throughput power. Note that the power throughput does not change significantly for the two fiber lengths.